ASYMMETRIES IN EXTRAGALACTIC RADIO SOURCES

ABSTRACT We investigated the correlations between asymmetric parameters – b (bending angle), x/Q (fractional arm-length ratio/arm-length ratio), and R* (lobe luminosity ratio) for FR-II extragalactic radio sources, our result indicates that quasars are more asymmetric than galaxies. Using the parameter , introduced by [1], we obtained result which indicate that the radial pressure variation of quasars’ environment differ from galaxies, leading to different asymmetric properties; but our sample is tainted by selection effects (the median redshift for galaxies in our sample is , while the median redshift for quasars is ), thus the difference is radial pressure variation is expected. KEY WORDS : Asymmetry, Quasars, Galaxies, Statistics – correlation INTRODUCTION Generally, large scale FR II and FR I [2] Extragalactic Radio Source (EGRS), shows large scale symmetries in sizes, shapes and radio luminosities, but the jets of FR II radio sources are always asymmetric, especially FR II radio-loud quasars. Another class of EGRS that shows jet asymmetry is Compact Steep Spectrum radio sources with their one-sided jet-core structure [3]. The observed radio radiations from the lobes of Active Galactic Nuclei (AGN) have been modeled as bifurcated ejection of energy from its nuclei in opposite directed channels [4, 5]. It is assumed that in the rest frame of the AGN, energy is released symmetrically on both sides and two identical beams are created. From the observed similarity and roughly symmetric disposition of the twin lobes about the core, many different forms of departures from this similarity have been observed. Thus, any asymmetric appearance of structure of EGRS is determined by source orientation with respect to the observer, as well as by any intrinsic difference (different starting times, velocities and different evolution of jet possibly induced by different environmental forces acting on the jet) between them. Investigation of the systematic and correlations between these various forms of asymmetries are necessary for understanding the physical model of Extragalactic Radio Sources (EGRS). We observe only the luminous part of the matter content in any structure; hence in describing asymmetry, we look for differences between position and brightness of corresponding elements on both sides of the core of EGRS [6]. Since observations provide us with 2-dimensional projection of the 3-dimensional EGRS, these asymmetries can be caused by various physical mechanisms – strong anisotropy in the core [7]; Doppler beaming and time delay effects [8, 9]; different distances to different parts of the radio structure [10]. One may follow [11] to summarize the factors determining asymmetries of classical double radio sources to include – intrinsic asymmetry in ejection mechanism; evolutionary asymmetry – for relativistic expansion speed of hotspots, the approaching hotspot will be seen at a later stage in its evolution; environmental – The medium through which the hotspots expands will affect its length and brightness distribution; and kinematic asymmetries arising from relativistic expansion of the hotspot [9,12] 43